Can I damage my transmission by using smaller tires?

Tiny
GARYW
  • MEMBER
  • 2000 SATURN SL1
  • 14,800 MILES
I installed 4 Blizaks that are 175/55-14. The standard tire is 185/55-14. The speedometer is now off by two miles an hour. If I am doing 35 it is really 33 MPH. The automatic transmission is also shifting differently. It takes longer to shift at 40 MPH but my real concern is a hard downshift which I believe is into 2 gear. Can I damage the transmission by leaving these tires on? To add to the confusing, rigth after I installed the tires the alternator failed and the car died when I stopped at a light. There was a big thud as I was getting on the freeway and it felt like the transmission. I thought it was the alternator because the car died a short time later. I don't want to buy new tires if it will not fix the problem.
Thank you in advance for your help.
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Tuesday, January 17th, 2012 AT 10:41 PM

23 Replies

Tiny
PROAUTOTECH
  • EXPERT
Your tires are 10mm narrower in width. The first number is the width. The second number is a fraction of the width for sidewall height. I don't think this small of a difference should cause any problems. Did you confirm that the alternator was the problem, I don't think this was related in any way either. Also, how did you calculte the 2 mph difference? No speedometers are actually dead on. They usually tell you you're going faster than you are. Let me know. Thanks
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Tuesday, January 17th, 2012 AT 11:00 PM
Tiny
GARYW
  • MEMBER
My brother owns a service station and we checked the alternator after my battery light came on. I ordered the alternator but did not get it installed until after the car died. My GPS shows how fast the car is going and I also checked it against police traffic monitors. This is a 2000 Saturn SL1. Thank you
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Wednesday, January 18th, 2012 AT 12:15 AM
Tiny
SATURNTECH9
  • EXPERT
Just to add to this one a bad alternator will greatly effect the transmission since its computer controled and its pressure is controled electrically. That car didnt come with 55 seris tires stock. So does the car still hessatiate to shift and have a harsh down shift into second since you changed the alternator?
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Wednesday, January 18th, 2012 AT 12:38 AM
Tiny
GARYW
  • MEMBER
Sorry, my mistake. The original tires were 185/65-14 and the current tires are 175/65-14. I did not notice any problems with the shifting until after the alternator was installed. It was also around the same time that I installed the tires. The down shifting seemed more noticeable today and that is why I submitted my question.
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Wednesday, January 18th, 2012 AT 12:55 AM
Tiny
SATURNTECH9
  • EXPERT
The tireds have nothing to do with the car shifting. Any delays going into reverse?So we know the new alternator is charging?
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Wednesday, January 18th, 2012 AT 2:26 AM
Tiny
GARYW
  • MEMBER
No problems shifting into reverse. The alternator was installed on Dec 1 and the car is used everyday. The last shift is around 40 mph but it acts like the cars did in the 1980s when they would shift in and out of the overdrive at a certain speed. If I stay at 39 mph it does not seem like it will shift. The RPM is just over 2,000.
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Wednesday, January 18th, 2012 AT 12:42 PM
Tiny
SATURNTECH9
  • EXPERT
Those transmission are smart they use a computer and will shift at all different times depending on how you drive them and whoses driving them. Its called adaptable shifting. It learns peoples driving and if you have more then one driver it will switch to other persons driving when they drive. Have you made sure the alternator is charging?So it still has a hard second gear down shift?Is this hard second gear down shift only under certain driving conditions?
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Wednesday, January 18th, 2012 AT 5:57 PM
Tiny
GARYW
  • MEMBER
I am the primary driver. Once in a great while my wife may use the car but I drive at least 50 miles a day for work. I did not check the charging system with the gauge but I suspect that my brother did when he installed the alternator. The lights are bright and the battery light is not on. I suspect that the battery would have died by now if it was not charging but I will check it this weekend. I noticed the hard down shift on my way home from work yesterday. It was a normal day and the same route that I typically drive. This is my biggest concern not the late up shift.
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Wednesday, January 18th, 2012 AT 6:32 PM
Tiny
SATURNTECH9
  • EXPERT
A late upshift and hard down shift are signs of too low and too high of pressure. Those cars have a lot of issues with the pressure regulators in the valve bodys causing too high and too low of pressure. At some point in the S models life they will all need the pressure regulators issues addressed.
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Wednesday, January 18th, 2012 AT 6:42 PM
Tiny
CADIEMAN
  • MEMBER
The only problem here is the speedo and the smaller tires causes a rpm change. This may cause this kinda problem.
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Wednesday, January 18th, 2012 AT 6:55 PM
Tiny
SATURNTECH9
  • EXPERT
There is barely a tire difference here I have seen a lot bigger tire differences put on these cars and never cause any shifting compliants.
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Wednesday, January 18th, 2012 AT 7:04 PM
Tiny
PROAUTOTECH
  • EXPERT
I agree with saturn tech9, I think you are witnessing a coincidence here. Or, possibly a vacuum line or something along those lines was disturbed or cracked during the install of the alternator. I would forget about the tires and look elsewhere.
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Wednesday, January 18th, 2012 AT 7:10 PM
Tiny
SATURNTECH9
  • EXPERT
The only vaccuum line I have seen effect shifting on those cars is the vaccuum hose that goes from the intake manifold to the pcv in the valve cover. See if the hose is collasped or has a hole in it if it does replace the hose. But the main reason for issues with those transmissions is a pressure issue like I was saying before. The only vaccuum lines by the alternator which you would have to go out of your way to hit them while changing the alternator is the evap canister purge solenoid.I have seen those hoses off and people still never mentioned a transmission issue.
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Wednesday, January 18th, 2012 AT 7:18 PM
Tiny
PROAUTOTECH
  • EXPERT
I know I was reaching there, but people sometimes unknowingly rest thier hands on things that can cause issues.
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Wednesday, January 18th, 2012 AT 7:24 PM
Tiny
SATURNTECH9
  • EXPERT
I know thanks for the input.
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Wednesday, January 18th, 2012 AT 8:01 PM
Tiny
GARYW
  • MEMBER
Thank you for all your help. My brother installed the alternator ans he has been a mechanic for over 40 years. I will look to see if anything has been disturb but wouldn't the idle be higher if there was a vacuum leak? There is no hole in the short PCV hose. It is strange but it did not feel as harse of a down shift on my drive home today. I did check the tire sizes on tire rack and the dia is pretty close on the two different tires.
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Wednesday, January 18th, 2012 AT 11:42 PM
Tiny
SATURNTECH9
  • EXPERT
I was saying the only vacuum hose leak issue I have seen cause a shifting issue was that pcv hose issue. If its not colasped or has any holes in it then its good. Also the transmission could just be relearning the adaptives since the alternator failed.
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Thursday, January 19th, 2012 AT 6:33 AM
Tiny
GARYW
  • MEMBER
Do you have any idea on how long it would take for the computer to relearn? The alternator was changed on Dec 1 and I usually drive it back and forth to work every day.I was going to disconnect the battery and reconnect it before I started these series of questions. I thought it might need to relearn again. I have yet to do that and again this morning I did not notice a hard down shift. The long shift into overdrive is still there.
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Thursday, January 19th, 2012 AT 7:38 PM
Tiny
SATURNTECH9
  • EXPERT
It should have relearned a long time ago the long shift into overdrive is a sign of low line pressure. Does your brother have a transmission pressure gauge set?Find out so we can do some pressure tests on the transmission to rule that out.
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Thursday, January 19th, 2012 AT 10:33 PM
Tiny
S,CLAY
  • MEMBER
I have a simular problem with my automatic 98 Subaru Legacy Outback. Just bought it at auction for $1200 and it has 209k miles but is in great shape. It needed tires so I replaced what it had on it 205 55 16's that I found used. It came with 205 70 15, did not realize this until I was trying to figure what the deal was with it's shifting problem. When you accelerate it shifts from 2nd to 4th then to 3rd, unless you give it hell. If I keep the petal to the metal it shifts great. I pulled the pan off the trans and changed the fluid, it was very clean. The diff in size is over 2" in DIA so could this be my shifting problem? Can I reset the ecm on this car by removing the battery cable?
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Sunday, May 13th, 2012 AT 3:40 PM

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